Thin plate-shaped materials such as semiconductor wafers and magnetic disks (hereinafter collectively referred to as wafers) are polished to a predetermined thickness by grinding or lapping processing, and are then mirror-finished by polishing processing.
In surface machining to obtain a surface with a high degree of flatness, the machining in which grinding processing, lapping processing, polishing processing, and the like, are performed, a single-side polishing apparatus which presses a wafer against a lower turn table with a polishing head and performs polishing while supplying polishing slurry or a double-side polishing apparatus which sandwiches a disk-shaped wafer between upper and lower turn tables and polishes both surfaces simultaneously while supplying polishing slurry is used.
FIG. 4 depicts an outline of the structure of an example of a conventional double-side polishing apparatus. A double-side polishing apparatus 81 mainly includes rotatable disk-shaped lower turn table 82 and upper turn table 83, between the upper and lower turn tables 82 and 83, a carrier plate 86 which has a wafer holding hole 86a holding a wafer W and performs planetary movement, and a sun gear 84 and an internal gear 85 which move the carrier plate. Moreover, an upper cover 91 covering an upper part of the apparatus is sometimes placed. In a polishing process, the wafer W is held between the upper and lower turn tables 82 and 83 each having a front surface to which a polishing pad is attached, and polishing slurry is supplied to the machining surfaces while rotating the wafer W and the upper and lower turn tables 82 and 83 and applying pressure from the side of the upper turn table 83, whereby the front surface of the wafer is gradually removed by the combined action of a chemical action and a mechanical action of the polishing slurry, and the purpose thereof is to provide an object to be polished with a mirror-finished surface and to flatten it. The above-described lower turn table 82, upper turn table 83, sun gear 84, and internal gear 85 are driven by their respective drive motors 88a, 88b, 88c, and 88d and speed reducers 89a, 89b, 89c, and 89d placed in an apparatus main body box 87. Although there is an apparatus of the type, which moves them with a single motor, in recent years a four-way type apparatus in which they are driven by their respective motors has often been used. In addition, in the box 87, as a means of circulating the air inside the box, an exhaust duct 92, an outside air intake 93, and the like, are placed.
When a silicon wafer is polished as a wafer W, since the polishing rate of the silicon wafer varies depending on the pressure which the wafer receives from the upper and lower turn tables and the temperature during polishing, in order to machine it to a wafer having a high degree of flatness, it is necessary to keep the shape of the polishing turn table and the temperature of the wafer machining surface constant. Therefore, in a common double-side polishing apparatus, the temperature of the wafer machining surface during polishing is maintained constant by controlling the amount and temperature of polishing slurry supplied to the wafer polishing surface so as to be constant, and additionally thermal deformation of the turn table is prevented by removing heat of polishing generated by machining by supplying cooling water at a constant temperature to a cooling jacket 90 placed in the turn table, whereby the pressure which the wafer receives from the upper and lower turn tables is made constant.
In recent years, the required precision of the flatness of the wafer has become higher. When the required precision of the flatness of the wafer becomes high as described above, the temperature control by the polishing slurry and the removal of heat of polishing generated by machining by supplying cooling water to the cooling jacket are not enough. In particular, during a few batches after the start of machining, degradation of the flatness of the machined wafer is noticeable, and the stability thereof is poor.
Furthermore, since slurry exchange, polishing pad dressing, or the like, consumes downtime of the apparatus on a regular basis even during continuous operation of the polishing apparatus, a change in the shape of the machined wafer is inevitable.
Moreover, as described in Japanese Unexamined Patent Publication (Kokai) No. 11-188613, a double-side polishing apparatus in which an auxiliary heating element is placed on a side of a turn table, the side opposite to the machining surface thereof, and the turn table is heated in advance before the operation of the apparatus and is also heated during operation, whereby the temperature of the turn table is maintained constant has been proposed. However, since it attempts to maintain the temperature of the turn table constant by heating, the flatness of the obtained wafer and the stability thereof are not satisfactory.